This invention is in the field of network communications. Embodiments of this invention are directed to synchronization of network devices in a power line communication network.
For many years, electrical utilities determined customers' usage of power over each billing period by way of a human visually reading dials or displays on a power meter at customer locations. This task consumes personnel and vehicle resources, is prone to human error, and provides only the periodic information of power usage since the previous reading. In recent years, the meter reading task has been made more efficient by the deployment of meters with wireless transmission capability, allowing the human meter reader to read the current meter state “from the curb”, greatly improving the efficiency of the process and enabling more flexible placement of the meter (e.g., in a secure location). However, the information available by this wireless approach is still limited to periodic snapshots of the current meter state.
In today's economy especially, both industrial and residential power consumers have become more aware and concerned over the cost of the electrical power that they consume. Utilities have experienced ever-increasing costs of adding production capacity, and as such are taking action to reduce peak demand levels, for example during late afternoon on hot summer days. In many markets, the electrical power industry has been deregulated to some extent, placing competitive pressure on retailers of electrical power. The mutual concern over electrical energy usage by utilities and their customers has provided an incentive for both to consider the implementation of “smart grid” technologies, including such actions as demand-based pricing, control of customer power usage from the utilities (in exchange for a reduced rate), remote control of power usage by the customer (e.g., lighting control, building automation, etc.) and the like. But these technologies require more frequent communication between the power meter at the consumer and the utility than the monthly meter reading, and require bidirectional communication between the meter and the utility.
Various technologies for power line communication (“PLC”) have been proposed and implemented. These PLC technologies involve the modulation of the alternating current (AC) power sinusoid with higher frequency signals bearing the information payload. This approach avoids the need for installation of an additional communications facility, such as twisted-pair or coaxial wire, and longer-range wireless transceivers. Because the volume of information to be transmitted is not overwhelmingly large, conventional PLC technologies are implemented as low-frequency narrow-band power line communications (LF NB PLC), which can be implemented at relatively low cost, requires relatively low power, and is bandwidth-efficient. The PLC communications can be extended into a local area network within the consumer location, enabling monitoring and control of thermostats, motor-driven equipment, solar panels, gas and water meters, and the like.
A recent approach to low-frequency narrow-band power line communications is known in the art as “PRIME” (“PoweRline Intelligent Metering Evolution”). PRIME PLC is an open standard defined in the physical (“PHY”) and media access control (“MAC”) protocol layers described in IEEE Standard 802.16. The PHY layer implements orthogonal frequency division multiplexing (OFDM) in the “CENELEC-A” frequency band, which is reserved in Europe for electricity suppliers. Under the current draft of the PRIME standard, namely “Draft Specification for PoweRline Intelligent Metering Evolution”, version R1.3.6, available from Prime Alliance TWG, 97 subchannels are assigned between 41992.1875 Hz and 88867.1875 Hz, one of those subchannels serving as a pilot subchannel and the others carrying bidirectional communications. These subchannels are modulated in OFDM fashion, according to a differential phase-shift-keying constellation (e.g., D8PSK). A PRIME subnetwork includes a single Base Node, typically at the transformer station for a neighborhood, and multiple Service Nodes in a tree topology for which the Base Node is the root node. Service Nodes serving as the “leaves” for the tree network (i.e., those Service Nodes that are not supporting a downstream node) are in a Terminal state, while Service Nodes at branch points in the tree structure are in a Switch state. Service Nodes begin in a disconnected state, and request promotion first to the Terminal state, followed by promotion to a Switch state upon the logical connection of another Service Node to it.
As known in the art, PRIME communications are arranged in data frames, specifically in “superframes” that each consist of a group of frames (e.g., thirty-two frames). PRIME PLC communications are beacon-based, in the sense that the Base Node and each Switch Node periodically transmits a “beacon” at a time slot at the beginning of particular frames within the superframe, as assigned by the Base Node. Each Switch Node retransmits all received traffic frames, transmitting its beacon in those frames in which it has been assigned a beacon slot. Frames received at a Switch Node other than its assigned frames (i.e., traffic in branches of the tree network to which the Switch Node does not belong) are not retransmitted by that Switch Node, to optimize traffic flow. Each Service Node in the tree network is able to sense the start of each frame, to synchronize communications.
The PRIME PLC standard and specification can be applied to either single-phase or three-phase power distribution networks. In the three-phase implementation, it is permitted for the Base Node to inject the PLC signal on all three phases, but the relevant specification significantly limits the signal power in that case. As such, Base Nodes in conventional PRIME networks typically inject the communications signal to only one of the three available phases, as a modulated signal on the selected phase relative to the neutral. By virtue of the close proximity of the power line conductors for the three phases, the injected PLC signal parasitically couples (i.e., capacitively, inductively, or both) to the other two phases. The parasitically coupled signals at the other two phases are necessarily attenuated relative to the injected signal. The extent of the attenuation can vary widely, but is typically being on the order of 10 dB.
Typical PLC Service Nodes are implemented as modems deployed at the customer location that are connected to only one phase of the three-phase power distribution network. This single phase may be the only phase deployed to the customer location (in the case where the customer does not have any three-phase equipment to be powered), or may be one of the three available phases received at that location. If the phase to which the Service Node is coupled is not the phase to which the Base Node PLC signal is directly injected, the Service Node will receive a less-than-optimal signal, due to the attenuation of the parasitic coupling between phases.
By way of further background, copending and commonly assigned U.S. patent application Ser. No. 13/531,324, filed Jun. 22, 2012, entitled “Beacon Selection in Communication Networks”, and incorporated herein by this reference, describes a communications network and method of operating the same in which a new Service Node joining an existing PRIME network associates itself with a selected one of a plurality of Switch Nodes. As described therein, each Service Node can “see” traffic from multiple Switch Nodes, and can, on joining the network, select one of those Switch Nodes (including the Base Node) to which to register itself. Application Ser. No. 13/531,324 describes several methods for selecting a Switch Node during registration of a new Service Node.